DISCONTINUED OPERATIONS AND SIGNIFICANT DISPOSALS - Significant Disposals (Details) loan in Thousands, $ in Millions			12 Months Ended
		Sep. 21, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) loan	Aug. 31, 2017 USD ($)	Mar. 31, 2017 USD ($) employee branch
Discontinued operations
Pre-tax gain (loss) on sale	[1]		$ 0	$ 247	$ 602
Mexico Asset Management Business | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Discontinued operations
Assets		$ 137
Liabilities		$ 41
Pre-tax gain (loss) on sale				250
After tax gain (loss) on sale of disposal				150
Term of services framework agreement		10 years
Income before taxes			0	123	164
Fixed Income Analytics and Index Businesses | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Discontinued operations
Assets						$ 112
Liabilities						18
Pre-tax gain (loss) on sale					580
After tax gain (loss) on sale of disposal					355
Goodwill						$ 72
US Mortgage Servicing Operations | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Discontinued operations
Pre-tax gain (loss) on sale					(331)
After tax gain (loss) on sale of disposal					(207)
Loss on disposal					382
Derecognition of servicing-related assets					1,162
Derecognition of mortgage servicing rights					$ 1,046
Number of loans to be disposed of | loan					750
Loans, net					$ 93,000
CitiFinancial Canada | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Discontinued operations
Assets							$ 1,900
Liabilities							1,500
Pre-tax gain (loss) on sale					350
After tax gain (loss) on sale of disposal					178
Income before taxes			$ 0	$ 0	$ 41
Loans, net							$ 1,700
Number of branches | branch							220
Number of employees | employee							1,400

[1] See Note 2 to the Consolidated Financial Statements for further information on significant disposals.